Cooh

Figure 7.16 The formation of tetraterpenes by condensation of two molecules of geranylgeranyl pyrophosphate, and conversion to carotenes, with some examples of insect carotenes, retinal or vitamin A and absicic acid, a degradation product of carotenes. The formation ofphytoene is very similar to that of squalene (Figure 7.4)

xanthophyll

Figure 7.16 The formation of tetraterpenes by condensation of two molecules of geranylgeranyl pyrophosphate, and conversion to carotenes, with some examples of insect carotenes, retinal or vitamin A and absicic acid, a degradation product of carotenes. The formation ofphytoene is very similar to that of squalene (Figure 7.4)

thought to be made only by plants, but that is now uncertain. Bovine corpus luteum has been shown to synthesize carotene. Carotenes are highly unsaturated and are very unstable in air, nevertheless they are found in many places, from butter to flamingos.

Lycopene is the red pigment of tomatoes, paprika and other fruit. (3-Carotene is an essential part of the photosynthetic process in plants. Cleavage of (3-carotene by oxidation gives two molecules of vitamin A aldehyde or retinal, an important part of the visual pigment of insects as well as higher animals. Other carotene derivatives give colour to many fruit and vegetables, e.g. zeaxanthin is the yellow pigment of maize (Zea mais). Astaxanthin is a further oxidation product of carotene and gives the pink colour of shrimps, boiled lobsters, salmon and the pink boll-worm. Violaxanthin is a common plant pigment, which on oxidation gives abscissic acid, a plant hormone that controls loss of leaves (Figure 7.16). It is also widely found in Lepidoptera. Lycopene, (3-carotene, green aphid pink aphid green aphid pink aphid

ß,y-carotene [major product]

Y,Y-carotene

ß.ß-carotene ß,cp-carotene torulene [major product]

3,4-didehydro-(p,c -carotene lycopene

Figure 7.17 The carotene pigments in the two colour forms o/Macrosiphum liriodendri aphids. The systematic names are used here. fi,fi-Carotene is more commonly known as P-carotene

ß,y-carotene [major product]

Y,Y-carotene

ß.ß-carotene ß,cp-carotene torulene [major product]

3,4-didehydro-(p,c -carotene lycopene

Figure 7.17 The carotene pigments in the two colour forms o/Macrosiphum liriodendri aphids. The systematic names are used here. fi,fi-Carotene is more commonly known as P-carotene zeaxanthin, violaxanthin, astaxanthin, xanthophyll and p-carotene monoepoxide and at least eight other carotenes have been found in insects. Xanthophyll, also known as lutein is found in almost all Lepi-doptera examined (the name xanthophyll is also used for all oxygenated carotenes). As retinal combines with the protein opsin to form the purple visual pigment rhodopsin, so carotenes combine with proteins to give green, blue-green, blue and red pigments in insect integument and haemolymph.

The carotenes of one aphid (Macrosiphum liriodendri), that occurs in two colour variants have been studied in detail. This aphid, living on tulip trees (Liriodendron tulipifera), exists in green and pink forms which appear to differ in their level of cyclizing enzymes. The green form have all cyclized carotenes, while the pink form have two partly cyclized and two uncyclized carotenes (Figure 7.17). It must be presumed the aphids have the necessary enzymes to cyclize lycopene. Many other insects, particularly aphids, contain carotenes along with other pigments.